This invention relates generally to medical imaging systems and more particularly to systems and methods for improving quality of an image.
A computed tomography (CT) imaging system typically includes an x-ray source that projects a fan-shaped x-ray beam through an object, such as a patient, being imaged to a multi-slice array of radiation detectors. The beam is collimated to lie within an X-Y plane, generally referred to as an “imaging plane”. Intensity of radiation from the beam received at a detector array is dependent upon attenuation of the beam by the object. Attenuation measurements from each detector element of the detector array are acquired separately to produce a transmission profile.
The x-ray source and the detector array are rotated within a gantry and around the object to be imaged so that a projection angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements and/or projection data, from the detector array at one gantry angle is referred to as a “view”. A “scan” of the object includes a set of views made at different projection angles. To perform a helical scan, a table controller moves a table, on which the object is located, parallel to an axis in synchronization with a rotation of the gantry, while the detector array collects the projection data.
One method for reconstructing an image from the projection data is a filtered back projection (FBP). The image is likely to be used for both soft-tissue pathology and bony structure investigation. FBP converts the projection data from a scan into an integer called a CT number or Hounsfield unit (HU), which is used to control brightness of a corresponding pixel on a cathode ray tube display. FBP provides good image quality and computational efficiency.
Iterative reconstruction (IR) is also used for the reconstruction of the image. An advantage of IR is that IR accurately models the projection data. The accurate modeling applies to the CT imaging system with the multi-slice detector array and capable of conducting the helical scan because the CT imaging system produces the projection data that pass obliquely through a plurality of two-dimensional (2-D) reconstructed image planes. By more accurately modeling the projection data, IR can produce reconstructions with higher quality, lower noise, and fewer artifacts. Although IR produces the image with significantly reduced noise in a soft-tissue region, the image in a bony region is generally not as sharp as the bony region in the image reconstructed using FBP. The difference in the sharpness is mainly caused by a factor, such as a nonlinear nature of a regularization, a dependence of a spatial resolution on a spatial location, an image frequency content, and a local contrast level. The factor may result in a lower detail or contrast in the bony region compared to the soft tissue region.